Apparatus for producing a radially uniform resistance characteristic in a semiconductor crystal by use of high frequency sonic vibrations

ABSTRACT

1. IN A METHOD OF PRODUCING A RADIALLY UNIFORM RESISTANCE CHARACTERISTIC IN A SEMICONDUCTOR CRYSTAL ROD THAT INCLUDES POSITIONING THE SEMICIONDUCTOR ROD IN A FLOATING MELT ZONE OPERATIONAL ENVIRONMENT AND SUBJECTING SAID ROD TO AXIALLY DIRECTED SONIC OSCILLATIONS, THE IMPROVEMENT COMPRISING, ELECTRICALLY GENERATING SAID SONIC OSCILLATIONS AT A FREQUENCY HIGHER THAN 20 KHZ. BY AN ELECTROMECHANICAL SOUND GENERATOR MEANS AND TRANSFERRING SAID SONIC OSCILLATIONS TO SAID ROD.   D R A W I N G

Oct. 22, 1974 w, KELLER ETAL 3,843,331

APPARATUS FOR PRODUCING A RADIAL-LY UNIFORM RESISTANCE CHARACTERISTIC INA SEMICONDUCTOR CRYSTAL BY USE OF HIGH FREQUENCY SONIC VIBHATIONS FiledDec. 1, 1972 3 Sheets-Sheet 1 ELECTROMECHANICAL SOUND GENERATOR MEANSFMZO' Fig.1

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Oct. 22, 1974 w. KELLER ETAL APPARATUS FOR PRODUCING A RADIALLY UNIFORMRESISTANCE CHARACTERISTIC IN A SEMICONDUCTOR CRYSTAL BY USE OF HIGHFREQUENCY SONIC YIBRATIONS 3 Sheets-Sheet 5 Filed Dec. 1, 1972 Fig.5

+528 POWER SUPPLY United States Patent O US. Cl. 23-301 SP ClaimsABSTRACT OF THE DISCLOSURE A method and apparatus for producing aradially uniform resistance characteristic in a semiconductor singlecrystal rod by subjecting a stock polycrystalline semiconductor rod to afloating melt zone purification and substantially simultaneouslycharging the stock rod with sonic oscillations in the rod axialdirection. The sonic oscillations are generated by an electromechanicalsound generator means, such as a piezoelectric sound generator means ora magnetostrictive sound generator means, at a frequency greater thankHz. The sound generator means is mounted directly on a rod holdingmeans and the high frequency sonic oscillations are transmitted to themelt zone of the rod by a coupling medium, such as composed of alead-tin alloy or a tetrafluoroethyleneepoxy resin, positioned betweenone end of the rod and adjacent rod mounting means. In certainembodiments, the electromechanical sound generator means is positionedat an angle to the rod axis so that the sonic oscillations travel in adirection forming a select angle up to 10 with the axis.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-partapplication of my copending application U.S. Ser. No. 283,320, filedAug. 24, 1972.

BACKGROUND OF THE INVENTION Field of the Invention The invention relatesto production of semiconductor crystals and more particularly to amethod and apparatus for producing a radially uniform resistancecharacteristic in a monocrystalline semiconductor rod.

Prior Art The production of monocrystalline semiconductor rods byfloating melt zone techniques is known. Generally, polycrystallinesemiconductor rods, such as of silicon, are transformed into single ormonocrystalline structures by placing a seed crystal at one end of thepolycrystalline stock rod and establishing an annular melt zone at suchend and then slowly moving the melt zone along the rod to the other endthereof. The re-solidified portion attains a higher degree ofcrystalline order and forms a single crystal. In such arrangements, thesemiconductor rod is generally vertically mounted between a pair ofmovable mounting means, one of which rotates about the axis of the rodduring the movement of the melt zone so as to assure a symmetricalgrowth of the re-solidified portion.

Single crystal rods produced by floating melt zone techniques exhibit aradially variable resistance characteristic, apparently because ofinsufficient mixing within the melt zone thereof. Of course, it ishighly desirable in the production of semiconductor components that thesemiconductor substrate have a radially uniform resistancecharacteristic.

3,843,331 Patented Oct. 22, 1974 My earlier application U.S. Ser. No.283,320 discloses a novel means of achieving improved mixing within amelt zone. The principles of that invention were demonstrated bycharging an unmixed dye solution with sonic oscillations to achieve asubstantially homogeneous solution. The application discloses that agood mixing within a melt zone can be achieved across a rodcross-section by charging at least one rod mounting means with audiosonic oscillations in the rod axial direction during the floating meltzone purification process. Rods treated in this manner exhibit a muchmore uniform radial resistance characteristic than conventionallyproduced semiconductor rods which have undergone a floating melt zonepurification. Nevertheless, it is desirable to achieve an improvedradially uniform resistance characteristic.

SUMMARY OF THE INVENTION The invention provides a novel method andapparatus for producing a monocrystalline semiconductor rodcharacterized by a radially uniform resistance characteristic.

It is a novel feature of the invention to electrically generate sonicoscillations at a frequency greater than 20 kHz. by means ofelectromechanical sound generator means and to transfer such highfrequency sonic oscillations to the melt Zone of a semiconductor crystalrod having a floating melt zone via a coupling medium positioned betweenthe stock rod and a rod mounting means.

It is a further novel feature of the invention to interconnect the meltzone of a semiconductor crystal rod with a cooled piezoelectric soundgenerator means or with a cooled magnetostrictive sound generator meansvia a rigid or muffling coupling medium.

It is another novel feature of the invention to position a soundgenerator means on a rod mounting means in such a manner that thegenerator sonic oscillations radiate in a direction that forms a selectangle with the semiconductor rod axis. In one embodiment, this angle isadjustable up to a maximum of 10 from the rod axis and in anotherembodiment up to a maximum of 1 from the rod axis. In a furtherembodiment, the sound generator is positioned to emit sonic waves in adirection that forms an angle to the rotational axis of thesemiconductor crystal rod and the: semiconductor crystal rod is mountedfor rotationally eccentric movement.

It is yet another novel feature of the invention to regulate theamplitude of sonic oscillations charged to the melt zone of asemiconductor crystal rod having a floating melt zone to a constantvalue as the melt zone moves along the rod by measuring the amplitude ofmodulation of the heating frequency (utilized to establish the meltzone) through the sonic oscillations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial elevated sectionalview of a semiconductor crystal rod being treated in accordance with theprinciples of the invention;

FIG. 2 is a partial sectional view of an embodiment constructed inaccordance with the principles of the invention;

FIG. 3 is a similar view of another embodiment constructed in accordancewith the principles of the invention;

FIG. 4 is a schematic illustration of a circuit utilized in accordancewith the principles of the invention for maintaining the amplitude ofsonic oscillations at a constant level within the floating melt zone;and

FIG. 5 is a schematic representation of a component of the circuitillustrated at FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention provides a meansof producing semiconductor crystal rods having a radially uniformresistance characteristic which is relatively simple, capable ofyielding reproducible results and producing high quality semiconductorcrystal rods.

Generally, the invention comprises subjecting a stock semiconductorcrystal rod to a floating melt zone purification and subjecting the meltzone to axially directed high frequency sonic oscillations. In thismanner, extremely good mixing action of the melt is always assured. Thematter within the melt zone is homogeneously distributed throughout thezone so that upon solidification thereof, a uniform or substantiallyuniform radial resistance characteristic through a cross-section of sucha crystal is attained.

Substantially all semiconductor materials can be subjected to thebeneficial processing of the invention. Silicon is an extremely commonsemiconductor material and a stock crystal thereof is readily processedin accordance with the invention to attain radially uniform resistancecharacteristics.

In accordance with the principles of the invention, sonic oscillationsare electrically generated through an electromechanical sound generatorat a frequency of more than 20 kHz. and transferred to a crystal rod viaa rigid or flexible coupling medium positioned between the rod and a rodmounting means.

Preferred electromechanical sound generator means include cooledmagnetostrictive sound generator means and cooled piezoelectric soundgenerator means that produce longitudinal sound waves in thesemiconductive crystal rod being processed. Such electromethanical soundgenerator means produce a higher crystal order within zonemelted singlecrystal rods in comparison to the crystal order achieved by my earlierdescribed invention. Density fluctuations due to crystallineimperfections that cause viscosity fluctuations during crystallinegrowth processes are decreased; also gas bubbles, gas bubbleagglomerates, density fluctuations, such as vacant lattice sites(Schottky and/or Frenkel defects) resulting from crystal growth, vacantlattice site agglomerations, doping in-homogeneities, which aresometimes referred to as striations, etc. are decreased in thesolidified matter. The mixing of the molten material is considerablyfavored in the proximity of the solidification front by the radiation ofsound waves. Accordingly, the radiation of sound waves can occur from aseed crystal or via the stock rod.

In a preferred embodiment, the electromechanical sound generator meansis positioned on a rod mounting means so that the direction of soundwaves produced thereby forms an angle with the rod axis. Preferably,this angle is adjustable up to a maximum of about from the rod axis.With such an arrangement of an electromechanical sound generator means,additional mixing effects are created because of the transversalcomponents in the sound waves.

The select electromechanical sound generator means is suitably energizedvia means, such as an electrical circuit, to produce sonic oscillationshaving a frequency in the range of 20 kHz. to 10 mHz. The select sonicfrequency may be continuously or irregularly varied in the region ofsound amplitude in an amount approximately greater than the period ofdensity fluctuation D and D (FIG. 1), wherein D is the period of coarsestriations and D is the period of fine striations. By correctly applyingsound radiations of 10 to 1000 w./cm. (watts per centimeter squared),sonic frequencies are produced that are in the kilohertz (kHz.) tomegahertz (mHz.) range. The vibration amplitude A is proportional to thedimension D, defined by the period of coarse striations D finestriations D and the melt zone length D The amplitude A of thevibrations is in all instances determined by the radiation power of aselect fixed frequency in accordance with wherein:

and provide the following results:

J A f 100, w./c1n. 20 m. 65 kHz.

100, w./cm. 200 pm. 0.5 kHz. 10-l00, w./cm. 20-200 pm. =5-10O kHz.

These conditions are more readily visualized by considering FIG. 1. Astock polycrystalline semiconductor rod 1, such as composed of silicon,undergoing a floating melt zone purification process is illustrated. Ofcourse, such a stock rod 1 is positioned in a melt zone purificationapparatus (not shown) which typically includes a housing connected to avacuum pump so that the interior chamber thereof is evacuated, a pair ofmovable mounting means extending from opposing walls thereof forsupporting the stock rod therebetween, a heating means, such as aninduction coil that encompasses a portion of the rod and defines a meltzone thereon, suitable energy sources and circuitry for transmitting thesame, means for selectively moving the rod within the housing, etc. Forthe sake of brevity, most of such conventional structure has beenomitted from the drawings and will be generally referred to in thespecification and claims as a floating melt zone operational environmentFMZ Referring back to FIG. 1, the rsolidified monocrystalline rodportion is designated by reference numeral 2; reference numeral 3designates a melt zone located between the rod mounting means (notshown) and reference numeral 4 designates an induction heating coilwhich provides a requisite heat to produce the melt zone. Referencenumeral 5 designates a sound generator means producing longitudinallyradiating sound waves in the direction of arrow 5a, which is generallyparallel with the rod axis.

The symbol D indicates the approximate melt zone length, generally about15 to 30 mm. (millimeters), the symbol D refers to the period of densityfluctuation wherein the coarse form of striations D are about to 200 am.(micrometers) and the fine form of striations D are about 5 to 20 m. Therequisite sonic power for effecting such density fluctuations is readilydetermined by the above formula (I).

In an embodiment of the invention, the amplitude of sonic oscillationscharged to the rod axis is regulated in relation to the position of themelt zone on the rod so as to be at a constant value within the meltzone. The sonic amplitude is regulated by measuring the amplitude ofmodulation of the induction coil heating frequency through the sonicoscillations. In this manner, a certain degree of independence betweenthe sonic oscillations charged to the rod and the location of the meltzone on the rod is achieved.

In exemplary embodiments, the coupling medium between the semiconductorrod and the rod mounting means is selected from the group consisting ofrigid and dampening or muflling mediums. Rigid mediums are preferablycomposed of metal and metal alloys, such as lead-tin alloys and themufiling mediums are preferably composed of an inorganic putty orplastic material, such as an epoxy resin modified withpolytetrafluoroethylene (Tefion-a registered trademark).

The coupling medium must interconnect the rod mounting means with therod over a uniformly large area in order to avoid high sonic powerdensities. The elastic characteristic of the coupling medium determinesthe amount of dampening and the phase location of the sonic wavesentering the melt. zone.

Crystals treated in accordance with the principles of the invention needno further treatment to achieve crystal order perfection. Dopingmaterial included during the growth of a crystal (radiallynon-homogeneously) is redistributed in a radially homogeneous manner.Components produced from such crystal rods have a higher quality due tothe more homogeneous starting materials as compared with crystalssubjected to more conventional dopant diffusion. For example, higherblocking voltages are more readily attained in semiconductor crystalstreated in accordance with the principles of the invention, which arenecessary for silicon raster vidicon structures.

Referring now to FIG. 2 wherein an upper portion of a stock rod in afloating melt zone operational environment FMZ is shown. The stock rod10 is composed of polycrystalline silicon and is mounted at its upperportion in a rod mounting means 11. A coupling medium 12, here shown asbeing composed of a plastic material, is positioned between the upperend of rod 10 and the adjacent surface of the mounting means 11. Inpreferred apparatus embodiments of the invention, the rod mounting means11 is mounted in a vacuum-tight and selectively movable manner on a wallof floating melt zone housing. Such mountings are conventional and thusnot shown. A sound generator means 13, which may be a water-cooledpiezoelectric sound generator means or a water-cooled magnetostrictivesound generator means is positioned above the mounting means 11 and indirect contact therewith. In order to increase the mixing action withinthe melt zone (not shown) and thereby diminish dopant fluctuationswithin the produced single crystal rod, the rod mounting means 111 isconnected with the sound generator means 13 in such a manner that thesound waves are radiated at an angle 17, up to a maximum of 10 to therod axis 18. The angle 17 is adjustable, for example, by varying theangles that the upper surface 11a of the mounting means 11 forms withthe lower surface 11b thereof. The thus created transversal sound wavescause an additional stirring or mixing effect within the melt zone. Amounting means 14 supports the sound generator means 13 and may berotationally driven by a power source (not shown) outside of thefloating melt zone housing.

FIG. 3 illustrates another embodiment of the invention and portionsthereof similar to portions in FIG. 2 have the same reference numerals.In this embodiment, the coupling media 12a is composed of a metal andthe rod mounting means 11 is provided with an upper surface 11a whichforms a smaller angle with the lower surface 11b thereof so that thesound waves generated by the sound generator means 13 radiate in adirection S that forms an angle 17 up to a maximum of 1 with the rodaxis '18. Of course, angle 17 is adjustable. The mounting means 14 maybe attached to a suitable drive means (not shown) to rotate the rod 10in an eccentric or wobbling movement to achieve further mixing actionwithin the melt zone.

FIG. 4 illustrates an operational circuit for maintaining the amplitudeof sonic oscillations at a constant level within a floating melt zone bymeasuring the amplitude of modulation of a heating frequency through thesonic oscillations. An induction heating coil 4 surrounds a portion of asemiconductor rod 10 so as to define a melt zone D thereon. A tuningcapacitor 20 is positioned Within the heating circuit supplying energyto the induction coil 4. The heating circuit includes a high frequencygenerator means 22, a decoupling coil 21 and a demodulator means whichinclude a demodulator diode 23 and an electronic filter 24 forseparating the ultrasonic modulation signal from the rectified highfrequency signal. An amplifier 25 receives the signal from the filter 24and transmits it to a desired actual-value comparison stage 26. Areference signal source 29 feeds an appropriate signal into stage 26 andstage 26 then feeds another ap propriate signal to controlling stage 27,which controls an ultrasonic power supply 28. The power supply 28transmits the required electrical energy to an electromechanical soundgenerator means 13 to produce the desired sonic oscillations.

FIG. 5 illustrates in some detail the circuitry of the desiredactual-value comparison stage 26 as well as its interconnection withother related components in the circuit shown at FIG. 4. The stage 26comprises a difference amplifier that is formed by two tubes 41 and 42and a common cathode resistance 43, which is grounded. A polarized relay44 is located in the anode branch of this circuit. The anode voltage issymmetrically guided to the tubes '41 and 42 via a coil 45 of thepolarized relay 44. The relay 44 includes a set of transfer contactswhich include stationary contacts 46 and 47 and a moving arm 48.

A desired signal is transmitted to the grid of tube 42 from thereference signal source 26- wherein the desired signal is produced by anadjustable potentiometer 50 and a voltage source 49. The referencesignal source is grounded to the difference amplifier of stage 26 at theside opposite that connected to the grid of tube 42. The grid of theother tube 41 receives the actual signal from the amplifier 25.Depending in which direction the actual signal deviates from the desiredsignal, either tube 41 or tube 42 directs current through the coil 45 ofthe relay 44 so that the switching arm 48 is moved either to contact 46or 47.

In the controlling stage 27, two motors 52 and 53 are coupled by thecontacts 46 and 47 respectively to an alternating current source 511,which supplies alternating electrical current to the motors. The motors52 and/or 53 readjust the ultrasonic current supply 28 to correspond tothe deviation of the actual signal from the desired signal in such amanner that the energy supplied by the ultrasonic current supply 28corresponds to the desired signal.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For example,the angle 17 may be adjusted by varying the relation of other surfaces,such as of the coupling media or of the sound generator means to thecrystal rod axis, to achieve the select angle between the sound wavedirection and the crystal rod axis. For this reason, it is to be fullyunderstood that all of the foregoing is intended to be merelyillustrative and is not to be construed or interpreted as beingrestrictive or otherwise limiting of the present invention, excepting asis set forth and defined in the hereto-appendant claims.

We claim as our invention:

1. In a method of producing a radially uniform resistance characteristicin a semiconductor crystal rod that includes positioning thesemiconductor rod in a floating melt zone operational environment andsubjecting said rod to axially directed sonic oscillations, theimprovement comprising, electrically generating said sonic oscillationsv at a frequency higher than 20 kHz. by an electromechanical soundgenerator means and transferring said sonic oscillations to said rod.

2. A method as defined in claim 1 wherein said sonic oscillations aregenerated at a frequency in the range of 20 kHz. to 10 mHz.

3. In a method as defined in claim 1 wherein said sonic oscillations areradiated in a direction which forms an angle with the axis of said rod.

4. In a method as defined in claim 1 wherein said sonic oscillations areradiated in a direction which forms a select angle up to 1 with the axisof said rod.

5. In a method as defined in claim 1 wherein said sonic oscillations areradiated in a direction which forms a select angle up to 10 with theaxis of said rod.

6. In a method as defined in claim 1 wherein said sonic oscillations areradiated in a direction which forms an angle with the axis of said rodand said rod is rotated through an eccentric path of travel.

7. In a method as defined in claim 1 wherein the amplitude of sonicoscillations is regulated in relation to the location of the melt zoneon said rod so as to maintain a constant value therein by measuring theamplitude of modulation of a heating frequency utilized to establishsaid melt zone through the sonic frequencies.

8. An apparatus for producing a radially uniform resistancecharacteristic in a semiconductor crystal rod comprising: an operationalenvironment for a floating melt zone purification process including apair of opposing crystal rod mounting means and an induction heatingcoil encompassing a portion of said rod to define a melt zone therein;an electrically energized electromechanical sound generator meanscapable of producing sonic oscillations with a frequency over 20 kHz.positioned outside said operational environment; a coupling mediumconnected at one end thereof to said electromechanical sound generatormeans and at the other end thereof to one of said pair of rod mountingmeans for transmitting said sonic oscillations to said rod, and meansfor energizing said electromechanical sound generator means.

9. An apparatus as defined in claim 8 wherein said electromechanicalsound generator means comprises a cooled piezoelectric sound generatormeans.

10. An apparatus as defined in claim 8 wherein said electromechanicalsound generator means comprises a cooled magnetostrictive soundgenerator means.

11. An apparatus as defined in claim 8 wherein said coupling medium iscomposed of a material selected from the group consisting of rigid andmuflling mediums.

12. An apparatus is defined in claim 11 wherein said coupling medium iscomposed of a material selected from the group consisting of metals,metal alloys and plastics.

13. In an apparatus for producing a radially uniform resistancecharacteristic in a semiconductor crystal rod, which includes anoperational environment for a floating melt zone process having a pairof opposing crystal rod mounting means, each of said mounting meanshaving an upper and lower surface, the improvement comprising: acoupling medium capable of transmitting sonic oscillations positioned indirect contact with an upper surface of said rod and a lower surface ofone of said rod mounting means, an electromechanical sound generatormeans capable of producing longitudinal sonic oscillations at afrequency in the range of 20 kHz. to 10 rnHz. positioned in directcontact with an upper surface of said one of said rod mounting means andmeans for energizing said electromechanical sound generator means.

14. In an apparatus as defined in claim 13 wherein said one of said rodmounting means is provided with an upper surface thereof that forms anangle with the lower surface thereof whereby said sonic oscillationsgenerated by said sound generator means in contact with such uppersurface radiate in a direction that forms an angle with the axis of saidcrystal rod.

15. In an apparatus as defined in claim 14 including a means for movingsaid rod in a rotationally eccentric path of travel.

References Cited UNITED STATES PATENTS 3,206,286 9/1965 Bennett 23273 SP3,216,805 11/1965 Emeis 23273 SP 2,952,722 9/1960 Jackson 23273 SP3,543,531 12/1970 Adams 23273 SP 3,340,016 9/1967 Wirth 23-273 SP3,637,439 1/1972 De Bie 23273 SP 3,592,937 7/1971 Emeis 23273 SP3,716,341 2/1973 Schmidt 23-273 SP 3,655,345 4/1972 Longo 23301 SP3,615,245 10/1971 Emeis 23-301 SP NORMAN YUDKOFF, Primary Examiner F.SEVER, Assistant Examiner US. Cl. X.R. 23273 SP

1. IN A METHOD OF PRODUCING A RADIALLY UNIFORM RESISTANCE CHARACTERISTIC IN A SEMICONDUCTOR CRYSTAL ROD THAT INCLUDES POSITIONING THE SEMICIONDUCTOR ROD IN A FLOATING MELT ZONE OPERATIONAL ENVIRONMENT AND SUBJECTING SAID ROD TO AXIALLY DIRECTED SONIC OSCILLATIONS, THE IMPROVEMENT COMPRISING, ELECTRICALLY GENERATING SAID SONIC OSCILLATIONS AT A FREQUENCY HIGHER THAN 20 KHZ. BY AN ELECTROMECHANICAL SOUND GENERATOR MEANS AND TRANSFERRING SAID SONIC OSCILLATIONS TO SAID ROD. 